1. Field of the Invention
The present invention relates to an image signal processing method capable of reproducing an image formed in consideration of visual psychology in a color copying machine or the like, and to an image signal processing apparatus capable of reproducing such an image.
2. Discussion of the Related Art
Color reproduction, tone reproduction, sharpness, and granule characteristics may be enumerated for key characteristics in discussing the quality of the output image of a color copying machine. Many image processing techniques for exactly reproducing an image on an original document have been proposed.
Incidentally, the final image receiving system in which an information transfer medium is a color output image is the visual perceiving system of a human being. Therefore, in designing an image signal processing system, it is desirable to handle the image quality in consideration with the visual property of the ophthalmencephalon on the basis of the effects of visual psychology.
This concept will be understood when reading a report that effective images result from an exact color design where the color correction process is carried out in the L* a* b* space as a color perception uniform space. The technique to carry out the color correction process in the uniform color space is disclosed in Japanese Patent Unexamined Publication No. Hei. 3-234178, for example.
However, the present technology dealing with the image noise, which determines the granularity of the image, continues in such a level that the consistency of the sharpness and the granularity is secured by cutting off the high frequency band of the sharpness emphasizing filter. Honjo et al. successfully leads the image noise/tone relationship from the visual characteristic, and reports the fact that the presence of noise effectively conceals tone steps, or so-called pseudo contour (S. Honjo & S. Taguchi; Paper Summaries of the 5th Intern. Congr. on Advances in Non-Impact Printing Technologies (SPSE), pp. 196 (1989 Nov. San Diego, Calif.). The basic concept of this technique has been widely used in the form of the dithering method for the half-tone generation.
Little has been proposed about the image signal processing technique handling the image noise, which takes cues of the visual psychology into consideration. Any decisive method to reduce only the image noise without deteriorating other image quality characteristics has not been proposed. In connection with this, let us consider a case of processing a noise-contained image on an original document, such as a generation image.
In order to improve the sharpness of the image, if the convolution operation of the unsharpness mask and the image is used for the sharpness emphasizing process, the sharpness is emphasized, but the image noise is also amplified, deteriorating the granularity quality of the image.
Attempts to reduce the noise produce an image of poor sharpness. The resultant image is a so-called "sleepy" image.
This unwanted phenomenon of the document image noise is observed also in the document image suffering from irregularity, tone steps (pseudo contour), and the like.